(1) Field of the Invention
The present invention relates to a method of estimating a state of charge of an on-vehicle battery that supplies electric power to on-vehicle loads, a method of estimating an open circuit voltage of the battery, and a method and device for computing a degradation degree of the battery.
(2) Description of the Related Art
Generally, as for an on-vehicle battery, an open circuit voltage upon fully charged state, an open circuit voltage upon completion of a discharge and an initial electrical quantity that is dischargeable from the open circuit voltage upon fully charged state to the open circuit voltage upon completion of a discharge are determined in advance with regard to a non-degraded battery. Then, a state of charge corresponding to any open circuit voltage or an open circuit voltage corresponding to a state of charge can be estimated on the basis of the open circuit voltage upon fully charged state, the open circuit voltage upon completion of a discharge and the initial electrical quantity.
As for a battery, its equivalent circuit is expressed by a series circuit of an electromotive force E0 and an internal impedance R0. One of parameters for expressing a state of the battery is a state of charge (SOC) as an initial electrical quantity that is dischargeable from the open circuit voltage upon fully charged state to the open circuit voltage upon completion of a discharge. The state of charge SOC is expressed by a percentage (%) as a relative capacity or ampere×hour (Ah) as an absolute capacity if an Ah value upon fully charged state is set to be 100%. The state of charge SOC can be estimated on the basis of an open circuit voltage (OCV), which is equal to the electromotive force E0 and is a terminal voltage in an open circuit state of a battery in its equilibrium state, in which state a polarization arisen in the battery due to a charge or discharge is canceled. Here, the open circuit voltage OCV has been measured or estimated.
If the SOC is expressed by a percentage, as seen in FIG. 35 illustrating a relation between SOC and OCV, the SOC (Ah) upon designing is always equal to an Ah converted value of the SOC (%), that is, a relation from the open circuit voltage upon fully charged state to the open circuit voltage upon completion of a discharge.
As for a battery, a reference internal (electrical) resistance value that varies depending on the state of charge is set clear as an internal impedance. The reference internal resistance value is used to know whether or not a terminal voltage of the battery becomes equal to or less than a vale of the open circuit voltage upon completion of a discharge when a certain discharge is carried out. For example, the reference internal resistance value is used to carry out a control of switching from a discharge to a charge when the open circuit voltage drops to a specific value or less.
However, when the battery degrades, the internal impedance tends to exceed the reference internal resistance value. In this case, the terminal voltage of the battery drops by a factor of an increase in the voltage drop in the battery during a discharge and therefore, a necessary terminal voltage of the battery cannot be obtained when a certain amount of discharge is carried out. For example, when a driver wants to start an engine of a vehicle again after the engine is once halted, the driver cannot start the engine again even if the driver thinks that the battery still has a sufficient capacity to start the engine.
That is, it is known that an allowable discharge capacity (ADC) of the battery decreases due to the increase in the internal impedance caused by the degradation. Accordingly, it has been regarded that the allowable discharge capacity should be revised by knowing the increase in the internal impedance caused by the degradation.
However, as mentioned above, even if the allowable discharge capacity is revised by knowing the increase in the internal impedance caused by the degradation, a necessary terminal voltage of the battery cannot be obtained when a certain amount of discharge is carried out, resulting in that the engine cannot be stated.
Facing such a problem as described above, so far one has understood the problem such that one could not correctly know the increase in the internal impedance caused by the degradation and one has solved the problem by taking a large margin for covering the inaccuracy. However, when the large margin is taken, it causes an obstruction when the capacity of the battery should be fully taken out. For example, in a hybrid vehicle, such a measure (taking a large margin) requires an early switching to a charge-direction, causing a degradation in fuel efficiency of a vehicle as a result.
As for a degradation of a battery, it has been known that inactivation degradation, that is, inactivation of active materials such as decrease in electrolyte and reduction in electrode-constituting material (thin plate made of lead in case of a lead battery) takes place besides the increase in the internal impedance as described above.
As shown in FIG. 35, in a case of the above-mentioned inactivation degradation, the relation between SOC and OCV is different from that in the case of non-degradation, in which the inactivation degradation does not take place, and the electrical quantity corresponding to the open circuit voltage upon fully charged state becomes small. To the contrary, in a case of the degradation, in which the internal impedance of a battery increases, its relation between SOC and OCV does not differ from that in a case of no degradation. As for a battery, in which the inactivation degradation takes place, if the OCV is converted into an Ah value, similarly to a case of a battery having no inactivation degradation, a value higher than its actual Ah value is obtained.
If such a value higher than its actual Ah value is obtained and therefore, if the accurate Ah value cannot be obtained, the OCV cannot be estimated accurately on the basis of the SOC, which varies from moment to moment during use of the battery and for example, when a driver wants to start an engine of a vehicle again after the engine is once halted, the driver cannot start the engine again even if the driver thinks that the battery still has a sufficient capacity to start the engine.